When discussing prosthetic materials, we often have in mind primarily those that are called main materials.

The main materials include the following: plastics, metals and their alloys, and ceramics.

In the field of dental prosthetics, plastics are often referred to as resins or artificial resins. Plastics are used for the production of both fixed and removable dentures. They are suitable for practically all types of these dentures.

At the present time, the most commonly used plastic, or resin, is acrylic polymer, most importantly polymerized methyl metacrylate. During polymerization, the molecules of the initial matter, or monomer, combine into macromolecular chains, called polymerizates. In the process, liquid methyl methacrylate (MMA), which is colourless but of repulsive odour, is transformed into clear, hard matter. Polymerization is only possible in the presence of one of the following agents: heat, ultraviolet light, or a chemical initiator. For the sake of completeness, it should be added that for use in dental prostheses, MMA is commonly polymerized with other materials to achieve better characteristics.

In order to facilitate the production process, a special procedure involving both liquid and solid form of methyl metacrylate is employed by the present-day technicians. Powdered PMMA spheres of specific sizes are mixed with MMA liquid monomer. Other agents according to a selected technology are applied. Susbsequently, special dough emerges which can be easily shaped by pressing, free modelling, or casting to become part of a prosthesis.

According to the end-product, polymerized plastics can be classified into two groups: crown plastics (resins), and denture base plastics (resins). Crown plastics are indicated for resin crowns, veneer of combined crowns, and ready-made artificial teeth. Denture base plastics are indicated for bases of removable dentures and orthodontic apparatuses.

There are several methods available to process polymerized plastics – free modelling, pressing, casting (injection), milling, and 3-D printing.

Other plastics are used in the dental laboratory, for example soft plastics. However, they are beyond the scope of this material.

Metals have been used in dental prosthetics since the very beginning.

However, pure metals (gold and titanium) are rarely used today. Instead, alloys of metals are used to achieve better properties of prosthetic materials.

Selected metals are melted together in various proportions and then cooled down so they become solid again (ie, they crystallize). Alloys with improved qualities can be used in different kinds of fixed and removable prosthetics.

One simple classification of alloys is as follows: i. Alloys containing noble metals, ii. Alloys from base metals.

Let us list a few commonly used base and precious metals together with their dinstictive features:

Cobalt

Chromium

Molybdenum

Nickel

Titanium

Gold

Palladium

Platinum

Silver

Ceramic materials have been used in dental restorations for quite some time - about two hundred years.

Thanks to these qualities, there has been a growing demand for them and new, improved materials have appeared frequently, especially since the emergence of CAD/CAM production technology. Ceramics are used to fabricate crowns, inlays, onlays, veneers, and frontal or lateral bridges.

Dental ceramics differ in composition from other industrial (regular) ceramics. They use very limited amounts of kaolin (more precisely, kaolinite), if any. Instead, they contain feldspar, a smaller amount of silica/quartz, flux, glazes, binders, pigments, and last, but not least, oxides and silicates – e.g. aluminium or zirconium oxides and lithium silicates – that improve the quality of ceramics by reducing brittleness. However, it must be noted that newer ceramics might be manufactured completely without some of the above mentioned elements.

After firing the components (various temperatures can be used), dental ceramic is composed of two so-called phases – an amorphous glassy phase and a dispersed crystalline phase. Traditional dental ceramics (feldspar-based) contain a higher proportion of the glassy phase which provides the desired high level of translucency. At the same time, they tend to be brittle, though. Therefore, the percentage of the crystalline phase is often increased on purpose (by adding leucite, alumina, spinel, zirconium or lithium disilicate) to achieve a stronger composition (while losing some of the translucency). In some allceramic materials, the percentage reaches as much as 90 per cent of the volume.

Besides all-ceramic restorations, another type is manufactured in dental labs: metal-ceramic. When the two materials are combined, the restoration can achieve the strength of metal alloys and the esthetics approximating those of all-ceramic restorations. A well-tested technique from this group is called porcelain-fused-to-metals (PFM). The ceramic component is fired onto the metal to produce hybrid crowns and fixed bridges with ceramic facets. A newer technique that has been pushing PFM out of the market is called pressed-to-metal (PTM) or pressed-over-metal (POM). Its advantage is a stronger and more esthetic ceramic.